Tuesday, September 16, 2014

A paper published today in Earth and Planetary Science Letters reconstructs the "El Niño-Southern Oscillation (ENSO) [which] represents the largest perturbation to the climate system on an inter-annual time scale" from a 10,000 year Holocene proxy in the Galapagos Islands.

According to the authors,

"Periods of strong or frequent El Niño tended to occur during peaks in solar activity and during extended droughts in the United States Great Plains linked to La Niña. These changing modes of ENSO activity at millennial and multi-centennial timescales may have been caused by variations in the seasonal receipts of solar radiation associated with the precession of the equinoxes and/or changes in solar activity, respectively.

El Niño and La Niña events are coupled in the Holocene

Intensification of both ENSO phases [El Nino & La Nina] broadly coincided with peaks in solar activity.Our data from the core of the ENSO region thus calls into question earlier studies that reported a lack of El Niño activity in the early Holocene. In agreement with other proxy evidence from the tropical Pacific, the mid-Holocene (5600–3500 yr BP) was a time of consistently weak El Niño activity, as were the Early Middle Ages (∼1000–1500 yr BP). El Niño activity was moderate to high during the remainder of the last 3500 years."

The paper joins many others in the scientific literature finding solar activity is the main driver of ENSO, as well as other ocean oscillations. Thus, solar influence on ENSO, "the largest perturbation to the climate system on an inter-annual time scale," is another of many solar amplification mechanisms described in the literature.

Alternating extremes in El Niño events characterize the early Holocene.

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Our data refute earlier studies on lack of El Niño activity in the early Holocene.

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El Niño and La Niña events are coupled in the Holocene.

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Intensification of both ENSO phases broadly coincided with peaks in solar activity.

Abstract

The El Niño-Southern Oscillation (ENSO) represents the largest perturbation to the climate system on an inter-annual time scale, but its evolution since the end of the last ice age remains debated due to the lack of unambiguous ENSO records lasting longer than a few centuries. Changes in the concentration and hydrogen isotope ratio of lipids produced by the green alga Botryococcus braunii, which blooms during El Niño rains in the Galápagos Islands, indicate that the early Holocene (9200–5600 yr BP) was characterized by alternating extremes in the intensity and/or frequency of El Niño events that lasted a century or more. Our data from the core of the ENSO region thus calls into question earlier studies that reported a lack of El Niño activity in the early Holocene. In agreement with other proxy evidence from the tropical Pacific, the mid-Holocene (5600–3500 yr BP) was a time of consistently weak El Niño activity, as were the Early Middle Ages (∼1000–1500 yr BP). El Niño activity was moderate to high during the remainder of the last 3500 years.Periods of strong or frequent El Niño tended to occur during peaks in solar activity and during extended droughts in the United States Great Plains linked to La Niña. These changing modes of ENSO activity at millennial and multi-centennial timescales may have been caused by variations in the seasonal receipts of solar radiation associated with the precession of the equinoxes and/or changes in solar activity, respectively.

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Also published today, a companion paper to the above paper showing El Nino strength peaked around 700-800 years ago and at the end of the record [left side of graph] was in the mid-range of the past 3000 years, i.e. not unprecedented or unusual:

Based on molecular and isotopic biomarkers from several types of plants and algae.

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Proxies used to infer past changes in mean rainfall and El Niño-related rainfall over last 3 kyr.

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Novel method used to infer changes in ITCZ-related rainfall during select periods.

Abstract

We present a 3000-yr rainfall reconstruction from the Galápagos Islands that is based on paired biomarker records from the sediment of El Junco Lake. Located in the eastern equatorial Pacific, the climate of the Galápagos Islands is governed by movements of the Intertropical Convergence Zone (ITCZ) and the El Niño-Southern Oscillation (ENSO). We use a novel method for reconstructing past ENSO- and ITCZ-related rainfall changes through analysis of molecular and isotopic biomarker records representing several types of plants and algae that grow under differing climatic conditions. We propose that δD values of dinosterol, a sterol produced by dinoflagellates, record changes in mean rainfall in El Junco Lake, while δD values of C34botryococcene, a hydrocarbon unique to the green alga Botryococcus braunii, record changes in rainfall associated with moderate-to-strong El Niño events. We use these proxies to infer changes in mean rainfall and El Niño-related rainfall over the past 3000 yr. During periods in which the inferred change in El Niño-related rainfall opposed the change in mean rainfall, we infer changes in the amount of ITCZ-related rainfall. Simulations with an idealized isotope hydrology model of El Junco Lake help illustrate the interpretation of these proxy reconstructions. Opposing changes in El Niño- and ITCZ-related rainfall appear to account for several of the largest inferred hydrologic changes in El Junco Lake. We propose that these reconstructions can be used to infer changes in frequency and/or intensity of El Niño events and changes in the position of the ITCZ in the eastern equatorial Pacific over the past 3000 yr. Comparison with El Junco Lake sediment grain size records indicates general agreement of inferred rainfall changes over the late Holocene.

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On the other hand, climate models and the IPCC have no consensus on ENSO due to climate change, likely because climate models do not consider any possible direct or indirect solar influence upon ENSO [from NOAA]:

When climate models are tasked with answering this question, they have struggled to give a consistent prognostication (Vecchi and Wittenberg 2010). For example, in Figure 2, the y-axis refers to ENSO variability (how frequently events occur) while the x-axis refers to the pattern of SST change (whether it looks more like El Niño or La Niña). While most climate models show a tendency towards more SST warming in central/eastern Equatorial Pacific than western (i.e. points located on the right side of the plot), there is no consensus on what that actually means for ENSO variations (i.e. points placement on the y-axis). Some models show decreasing variability (i.e. points in the bottom half), increasing variability (i.e. points in the upper half), or remains nearly the same (i.e. points in the middle). The point near the origin (center) would be if that bratty child in the dining room adjusted the switches but the total amount of light never changed.

Figure 2. Projections by 21 global climate models of changes in the mean state of sea level pressure of the tropical Pacific Ocean (x-axis) and ENSO variability (y-axis). When climate projections of the mean state of the tropical Pacific resemble something that looks like El Niño, points are greater than 0 along the x-axis and vice-versa for La Niña (points less than 0). When ENSO variability increases, points are located greater than 1 along the y-axis and vice versa for a decrease in ENSO variability (points less than 1). We see here that most climate models project a trend towards an El Niño-like average state of the tropical Pacific Ocean but there is considerable model-to-model disagreement regarding whether ENSO variability will increase, decrease, or remain the same in the future. Adapted from IPCC AR4 (2007).

What does the Intergovernmental Panel on Climate Change (IPCC) say?

It’s not surprising then that the IPCC report issued in 2013 takes a measured approach. The IPCC has LOW confidence in exactly what will happen to ENSO in the future even while they have HIGH confidence that ENSO itself will continue (IPCC, 2013).

Even though we are not sure how ENSO will change in the future, the impacts of ENSO will probably be affected. In a warming world, rainfall variability is expected to increase. Wet areas will likely get wetter, while dry areas get drier. Combined with a future ENSO event, climate change could strengthen or weaken the typical weather patterns associated with ENSO.

For instance, during El Niño events, below-average rain usually occurs across Indonesia. However, according to climate models, average annual rainfall will increase in the same area. So, during future El Niño events, dry conditions in Indonesia may not be as dry as today. Similarly, La Niña events are associated with a drying of southwestern North America and so are impacts from climate change. So drying during future La Niñas may be enhanced (Vecchi and Wittenberg 2010).

This is a very brief overview of the potential impact of climate change on ENSO and on how the atmosphere and ocean might change in the next hundred years or more. But remember, just because we do not have high confidence on how ENSO might change in the future does not mean that it won’t. It just means scientists have more work to do.